Method of focusing charged particles to provide zero momentum dispersion



June 23, 1964 BRQWN ETAL 3,138,706

METHOD OF FOCUSING CHARGED PARTICLES TO PROVIDE ZERO MOMENTUM DISPERSIONFiled April 28, 1961 INV EN TORS KARL L. BROWN WOLFGANG K. H. PANOFSKY yJOHN F. STPE/B PATENT AGENT United States Patent 3,138,706 METHOD OFFOCUSING CHARGED PARTICLES TO PROVIDE ZERO MOMENTUM DISPERSION Karl L.Brown, 1990 Santa Cruz St., Menlo Park, Calif.,

and Wolfgang K. H. Panot'sky, Los Altos, Calif., and

John F. Streib, Seattle, Wash; said Panofsky and said Streib assignorsto said Brown Filed Apr. 28, 1961, Ser. No. 106,331 5 Claims. (Cl.25041.9)

The present invention relates to the focusing and analysis of chargedparticles and primarily, though not exclusively, to methods of andapparatus for the focusing and analysis of secondary charged particlesresultant from bombardment of a target by high energy electron beams.

The bombardment of targets by high energy electron beams (cg. 600 mev.)initiates the emanation of secondary charged particles which leaves thetarget at divergent angles. Furthermore, the secondary particles havedifferent momentums, in many cases extending over a relatively largemomentum spectrum or band. In order to analyze these secondary chargedparticles, various magnetic and electric particle deflecting systemshave been devised. Presently available systems are capable of re-:ceiving monoenergetic particles emanating from the target source withina predetermined solid angle and can focus these monoenergetic particlesat substantially a point image where a suitable detector can count theparticles within a predetermined timed interval for purposes ofanalysis. However, particles of variant momentum traversing these samesystems will become dispersed thereby complicating the detection system.

Accordingly, it is the general object of the present invention toprovide a method of and apparatus for focusing charged particlesemanating from substantially a point source so that all particles have acommon focal point or image irrespective of their initial momentum ordirection from such source, thus, ultimately enabling simplifiedparticle analysis to be achieved.

More particularly, it is a feature of the invention to provide afocusing method wherein lateral deflection of the charged particles iseffected in a manner such that the radial and transverse foci coincideand the radial momentum dispersion at the common focus is zero.

A further feature of the invention is the provision of a focusing methodwhich can utilize magnetic or electric devices to establish therequisite particle deflecting forces.

Yet more particularly, it is a feature of the invention to provide afocusing method which, for various specific purposes, can utilize one ormore magnetic or electric devices to establish a particular deflectingforce pattern wherefore specifically desired particle trajectories canbe achieved.

Additionally, it is a feature to provide a particle focusing methodwherein particles of variant momentum traverse variantmomentum-determined paths between the source and the focal point so thatblocking of a certain segment of the particles at an intermediateposition of their trajectories will provide momentum selection.

Yet another feature of the invention is to provide apparatus foreffectively carrying out all of the above-mentioned features of theparticle focusing and analysis method embodying the present invention.

These as well as additional objects and features of the invention willbecome more apparent from a perusal of the following description andexplanation of the material depicted in the accompanying drawingwherein:

FIG. 1 is a radial ray'diagram of monoenergetic particle trajectoriesfrom substantially a point source or object through a focusing systemembodying the present invention so as to arrive at a common focal orimage point,

FIG. 2 is a transverse ray diagram of monoenergetic 3,l38,706 PatentedJune 23, 1964 particle trajectories through the same system from theobject to the image,

FIG. 3 is a radial ray diagram of the trajectories of particles with thesame angle of emission from the point source but with a finite spread ofmomentums,

FIG. 4 is a radial ray diagram of monoenergetic particle trajectories ina modified system embodying the present invention, and

FIG. 5 is a diagrammatic view of a particle analysis system embodyingthe features of the present invention and enabling the determination ofparticle mass.

With initial reference to FIG. 1, a particle P emanating from a source10 of charged particles is subjected to lateral deflecting forces so asto follow the curvilinear path or trajectory illustrated which defines acentral orbit. Other charged particles P and P each having the samemomentum as the particle P emanate from the same source 10 but atdivergent angles and are also subjected to lateral deflecting forces ofa nature such that all particles having the same momentum and emanatingfrom the same source 10 will be focused to a predetermined point imageindicated at 12.

In practice, the source 10 obviously has finite dimensions, but thesecan be maintained relatively small so that for purposes of the followingdiscussion, such source 10 can be considered as substantially a pointsource.

The mentioned lateral deflecting forces can be established by electricor magnetic devices, bending magnets of more or less conventional designbeing quite commonly employed for this purpose. As illustrated in FIG.1, two bending magnets 14, 16, or equivalent particle-deflectingdevices, are each arranged to bend the particle P traversing the centralorbit in the same sense through an angle of approximately so that atotal angular deflection of 220 is experienced in the entire trajectoryof this particle from the source 10 to the image 12. Additionally, inaccordance with the present invention, each magnet 14, 16 is arranged todeflect the divergent particles P P through a greater or lesser anglesuch that a radial crossover point 13 is established a predetermineddistance beyond the first magnet 14. Considering this radial crossoverpoint 18 as the center of the system, the magnets 14, 16, asillustrated, are arranged symmetrically on opposite sides thereof andare of substantially identical nature so that complete symmetry of thesystem is established. More particularly, in view of this symmetry, itcan be easily recognized that the divergent particle P will have a pathlength shorter than the path length of the central particle P betweenthe point source 10 and the crossover point 18, but will have an equallylonger path length than that of the central particle between suchcrossover point 18 and the final image 12. Additionally, the otherdivergent particle P will have a longer path length between the source10 and the radial crossover 18 than the path length of the centralparticle P but will, in turn, have a shorter path length between thecrossover point 18 and the final image point 12. In summary, it can beseen that in accordance with the present invention, all particles of thesame momentum leaving the point source 10 and subjected to thedeflecting forces of the magnets 14, 16 or other deflecting devices willhave the same path lengths between the point source 10 and the pointimage 12. It will be obvious that only particles within a predeterminedsolid angle of emission will be accepted by the system but that thislimitation is merely a practical and not a theoretical one.

If the system of FIG. 1 is viewed from the right and developed, thetransverse particle trajectories of monoenergetic particles will appearas shown in FIG. 2. The central particle P will pass directly from thesource 10 to the image 12 while divergent particles P R, will traversepaths which first diverge from the central path or trajectory andthereafter converge to arrive at the previously described image point12. Thus, the illustrated system provides for coincident foci of bothradially and transversely divergent rays of particles having the sameenergy or momentum. Various ways are known to establish this coincidenceof the transverse and radial foci. One manner is described by D. L. Juddin the Review of Scientific Instruments, vol. 21, p. 213 in 1950, and isgenerally known as gradient focusing. An alternative technique known aswedge focusing is described by K. T. Bainbridge in Experimental NuclearPhysics edited by E. Segre, published by John Wiley & Sons in 1953, vol.1, p. 559 if.

The manner in which radially and transversely divergent monoenergeticparticles are brought to a common focal point having been established,the manner in which zero momentum dispersion at such focal point may beprovided in accordance with the present invention must now beexplicated. It has been determined that the momentum dispersion ofparticles emanating from substantially a point source is defined by:

image f Sh dt object where:

t is the distance from the source or object measured along the centralorbit,

h is the curvature of a particle on the central orbit at position t, and

S is the deviation from the central orbit of a particle at position Iand having the same momentum as the central orbit particle.

Consequently, if this integral is equated at zero, it can be seen thatthe momentum dispersion at the image or focal point will be zero to thusprovide the necessary and sufficient condition for zero momentumdispersion.

With specific reference to FIG. 1, this necessary and sufiicientcondition for zero momentum dispersion at the image can be stated in asomewhat more physically obvious manner: if any two adjacent particlesof the same momentum have the same path lengths between the object andimage, then the system will have zero momentum dispersion. As haspreviously been explained with reference to the radial ray diagram ofFIG. 1, the path lengths of the various particles of the same momentumin the illustrated system do have the same path lengths and thus thenecessary and suflicient condition for zero dispersion at the image forthis particular system is established.

If the bending magnets 14 and 16 of FIG. 1 are employed, this necessaryand suflicient condition can be stated in yet another manner: if the netmagnetic flux enclosed by two adjacent rays of the same momentum betweenthe object and image is ZEIO, then the momentum dispersion at the imagewill also be zero. It should be noted that if the two adjacent rayscross, as illustrated in FIG. 1, such crossing is equivalent toreversing the sign of the enclosed flux.

If the necessary and sufiicient condition is established in a systemsuch as illustrated in FIG. 1, the paths or trajectories of particles ofvariant momentum will appear substantially as illustrated in FIG. 3. Asthere shown, three particles P P and P emanate from the point source 10in the same direction, but the particles P and P respectively, havegreater and lesser momentum than the central orbit particle P As aresult of their differing momentum, the deflecting forces of the firstmagnet 14 will cause a divergent of the particle paths or trajectories,but subsesequently, exposure to the deflecting forces of the secondmagnet 16 will have an opposite effect so that the particles will bebent by the deflecting forces into convergent paths so as to intersectat the radial focus or image 12. Thus, ultimately, it will be seen thatall charged particles from the point source 10 passing through thedescribed deflecting system will have a common focal point (image)irrespective of their initial momentum or direction from the pointsource.

Having thus established the conditions for coincidence of the radial andtransverse foci and zero momentum dispersion at such common focus, theimproved focusing methof of the present invention simply entailssubjecting charged particles emanating from a point source to lateraldeflecting forces such that monoenergetic particles are focused to apoint image and follow substantially curvilinear paths to satisfy thecondition that:

image f Shdt=0 object As specifically delineated in the system shown inFIGS. 1, 2, and 3, the method entails the steps of first subjecting thecharged particles emanating from substantially a point source to a firstlateral deflecting force such that the particles follow a curvilinearpath and monoenergetic particles pass through a radial crossover orintermediate focus and thereafter subjecting the particles to a secondlateral deflecting force of like quantity and sense to the first forceand symmetrically positioned relative to the crossover wherebymonoenergetic particles come to a second focus or image and the totalpath lengths of particles of equal momentum between the source and imageare equal, or in other words, the condition is satisfied that:

f shdt=0 object Such focusing method can, of course, be embodied inother systems. As one obvious modification, the two-magnet systemexplained hereinabove and illustrated in FIGS. 1, 2, and 3 can bereadily transformed to a single magnet system by the juncture, inprinciple, of the two magnets so that the radial crossover, illustratedin FIG. 1, will lie within the single magnet. As in the two magnetsystem, the single magnet can be designed so that the radial andtransverse foci coincide and the radial momentum dispersion at suchcommon focus will be zero. The focusing method consequently is preciselythe same in its broader aspects and differs only in details of thedeflecting force application.

An additional modified arrangement embodying the invention so as toincorporate the zero momentum dispersion focusing method is illustratedin FIG. 4 where two magnets 20, 22 or equivalent deflecting devices areutilized but are arranged to effect bending of the particles in oppositedirections or senses. In this arrangement, the magnets 20, 22 aresymmetrically arranged about a midplane, indicated at 24, but in orderto establish the zero dispersion requirement entailing equal pathlengths of monoenergetic particles emanating from the point source 26 invarious directions, the deflecting forces must be applied so that noradial crossover between the two magnets is experienced. The radial andtransverse foci can be made to coincide at a point image 28 inaccordance with the principles first devised by Judd or Bainbridge, andin accordance with the present invention, the deflecting forces can bearranged so that all of the path lengths of the various monoenergeticparticles will be equal to thus effect zero momentum dispersion at thecommon focal point or image 28, as visually illustrated in FIG. 4.

While not illustrated, it will be also obvious that more than twomagnets can be employed to embody the invention and carry out thedescribed focusing method so that the transverse and radial focicoincide and the momentum dispersion at such focal point is zero. It isto be particularly noted that in the illustrated systems, a plane ofsymmetry is established, but this symmetry condition is not essential tothe desired result of zero momentum dispersion as described. As long asthe described integral image f Shdt object is equated to zero, zeromomentum dispersion will be obtained. One additional feature may bementioned that may not be entirely obvious from the illustrated systems.Since no restriction is made on the position of either the object orimage, these points can obviously be positioned at infinity and as aresult, a parallel beam of particles entering the system will emerge asa parallel beam irrespective of the momentum of the individualparticles.

The described focusing method lends itself admirably to particleanalysis and by way of example of utilized in a mass spectrometerdiagrammatically illustrated in FIG. 5. Such spectrometer has beendescribed in detail in an article entitled, Double FocusingZero-Dispersion.Magnetic Spectrometer in the Review of ScientificInstruments, vol. 31, No. 5, pp. 556564 in May of 1960, and details ofits construction can be found in this article. Such spectrometer wasdesigned for use with a linear electron accelerator (not shown) Whosebeam 30 of electrons having an energy of approximately 600 mev. wasdirected against a target 32 to effect the emission of secondary chargedparticles P, whose analysis was desired. The electron beam 30 generatedby the accelerator consists of separated groups or bunches of electronsresulting from the characteristic operation of the accelerator which isenergized by radio frequency energy having a frequency of 2856megacycles. Thus, the emanation of secondary charged particles P occursat spaced intervals in time. The method of focusing these particlescorresponds to that described with regard to FIGS. 1, 2 and 3, andemploys two 110 bending magnets 34, 36 disposed symmetrically to bendthe particles P in the same sense and arranged to focus the particlesregardless of their direction from the target 32 or their initialmomentum to substantially a point image whereat a suitable particledetector 38 is located.

At the radial crossover between the two magnets 34, 36, members 40defining a. momentum-defining slit are disposed so that as a result ofthe momentum spread, best illustrated in FIG. 3, only particles within aselected momentum spectrum or band can pass therethrough and reach thedetector 38. Obviously, the position of the members 40 can be changed tovary the size of the slit wherefore more or fewer particles will beblocked and a narrower or broader momentum band thus be permitted toreach the detector 38, as the case may be.

Additionally, between the first magnet 34 and the members 40, theparticles are caused to pass through a resonant cavity 42 that isexcited by the same radio frequency energy that drives the acceleratoritself, wherefore electric fields are established in such cavity intimed relation to the emanation of charged particles P by thebombardment of the target 32 by the bunched beam 30 of electrons. Moreparticularly, radio frequency energy is fed into the cavity 42 so as toexcite the same in the T.M. mode that establishes electric fieldsadapted to transversely deflect particles P traversing the cavity anamount which depends upon their time of arrival therein. Particles Pleaving the target 32 at a desired velocity will suffer no transversedeflecting forces while particles of either greater or lesser velocitieswill be transversely deflected and will subsequently be blocked by themembers 40 of the previously described momentum selector. Consequently,this microwave cavity 42 operates as a velocity selector, permittingonly particles P of predetermined velocity to pass therethrough withoutdeflection so that they may subsequently traverse the slit and reach thedetector 38. A phase shifter 44 is placed in the radio frequency inputto the cavity 42 to enable establishment of the deflecting electricfields at the appropriate time so that the desired particle velocity isselected.

Since momentum and velocity selection are both achieved by thearrangement illustrated in FIG. 5, and momentum is equal to the productof mass and velocity, the precise measurement of velocity and momentumprovides for the ready determination of the particle mass.

From the foregoing description of the mass spectrometer of FIG. 5, itwill be understood that the previously described focusing method canwith but the addition of the steps of velocity and momentum selectionbecome a method for particle analysis. More particularly, the particleanalysis method entails the steps of the previously described focusingmethod and the addi tional step of periodically establishingtime-varying transverse deflecting forces in timed relation to theemanation of particles from the target and the further step of blockingthe traverse of a certain segment of the particles so that onlyparticles within a predetermined momentum and frequency range reach thedetector.

Obviously, various other modifications in the described focusing andanalysis methods and in the systems embodying those methods can be madewithout departing from the spirit of the present invention; andaccordingly, the foregoing description of certain specific arrangementsis to be considered as purely exemplary and not in a limiting sense; andthe actual scope of the invention is to be indicated by reference to theappended claims.

What is claimed is:

1. The method of focusing charged particles emanating from substantiallya point source which comprises subjecting the charged particles to afirst lateral deflecting force such that the particles followpredetermined substantially curvilinear paths, and thereafter subjectingthe charged particles to a second lateral deflecting force disposed inpredetermined relationship to the first force such that the particlesfollow additional predetermined curvilinear paths and monoenergeticparticles are focused to a point image and the condition is satisfiedthat:

f shd: 0

object wherein t is the distance from the source or object measuredalong the central orbit, h is the curvature of a particle on the centralorbit at position t, and S is the deviation from the central orbit of aparticle at position t and having the same momentum as the central orbitparticle, wherefore the momentum dispersion at the image is zero.

2. The method of focusing charged particles according to claim 1 whereinthe first and second deflecting forces are equal and are symmetricalabout a predetermined plane therebetween.

3. The method of focusing charged particles according to claim 1 whereinthe deflecting forces bend the particle trajectories in the samedirection or sense.

4. The method of focusing charged particles according to claim 1 whereinthe deflecting forces bend the particle trajectories in the oppositesense.

5. The method of focusing charged particles emanating from substantiallya point source which comprises subjecting the charged particles to afirst lateral deflecting force such that the particles followpredetermined curvilinear paths and monoenergetic particles come to anintermediate focus, and thereafter subjecting the charged particles to asecond lateral deflecting force of like quantity and sense to the firstforce and symmetrically positioned relative to the first force about theintermediate focus whereby monoenergetic particles come to a secondfocus or image, the entire particle paths being such that the conditionis satisfied that:

and having the same momentum as the central orbit particle.

References Cited in the file of this patent UNITED STATES PATENTS WhiteAug. 23, 1960 Leboutet et a1 Apr. 24, 1962 Marshall Sept. 25, 1962

1. THE METHOD OF FOCUSING CHARGED PARTICLES EMANATING FROM SUBSTANTIALLYA POINT SOURCE WHICH COMPRISES SUBJECTING THE CHARGED PARTICLES TO AFIRST LATERAL DEFLECTING FORCE SUCH THAT THE PARTICLES FOLLOWPREDETERMINED SUBSTANTIALLY CURVILINEAR PATHS, AND THEREAFTER SUBJECTINGTHE CHARGED PARTICLES TO A SECOND LATERAL DEFLECTING FORCE DISPOSED INPREDETERMINED RELATIONSHIP TO THE FIRST FORCE SUCH THAT THE PARTICLESFOLLOW ADDITIONAL PREDETERMINED CURVILINEAR PATHS AND MONOENERGETICPARTICLES ARE FOCUSED TO A POINT IMAGE AND THE CONDITION IS SATISFIEDTHAT: